This invention relates to bromobutyl rubber, and more particularly to bromobutyl rubber of improved green strength and methods for its preparation.
Butyl rubber is well known. The commercially available rubber comprises a copolymer of isobutylene and isoprene, the amount of isoprene being from about one half of about three weight percent of the copolymer. It is highly inert, chemically resistant, rubbery polymer which can be compounded and cured to give synthetic rubber of outstanding air impermeability, useful in making tire inner tubes.
Bromobutyl is also well known. It may be prepared by treating a solution of butyl rubber, in an organic solvent, with bromine and recovering the bromobutyl by contacting it with steam and drying the resulting aqueous slurry. Bromobutyl may contain up to 3 bromine atoms per carbon-carbon double bond originally present in the polymer or expressed another way, from 0.5 to 15 weight percent of bromine. A preferred bromobutyl contains about one bromine atom per carbon-carbon double bond originally present in the polymer or about 1 to 5 weight percent of bromine and a most preferred bromobutyl contains from about 1.5 to about 3 wt. percent bromine. Bromobutyl has a greater degree of reactivity than butyl, so that it can be blended with other unsaturated polymers and co-vulcanized therewith, which the unreactivity of butyl precludes. Bromobutyl vulcanizates, however, show good air impermeability, heat aging characteristics and general chemical resistance. It finds one of its principal uses in the tubeless tire inner liners. Such liners are in effect thin sheets of rubber, adhered to the tire carcass by co-vulcanization with the rubbers comprising the tire carcass. The heat aging characteristics, air impermeability and co-vulcanizability of bromobutyl render it suitable for use in such tire inner liners. Other known uses for bromobutyl include tire white sidewall compounds, heat resistant tubes and bladders.
A deficiency of bromobutyl is its lack of green strength. Green strength is a term applied to denote the strength, cohesiveness and dimensional stability of rubber compounds before they are vulcanized or cured. Lack of green strength renders difficult the processing and moulding of rubber compounds based on bromobutyl. For example, in the manufacture of tire liners, very thin sheets of bromobutyl rubber compound have to be prepared, applied to the green tire carcass and then cured. If the bromobutyl rubber compound is deficient in green strength, there is risk of rupturing the thin sheets unless very careful handling thereof is undertaken.